Determination of adapalene in gel formulation by conventional and derivative synchronous fluorimetric approaches. Application to stability studies and in vitro diffusion test

Background: Adapalene is a retinoid analogue with actions similar to those of tretinoin. It is used in topical treatment of mild to moderate acne. A survey of the literature reveals that no spectrofluorimetric method has been reported yet for determination of ADP, so it was thought necessary to develop a highly sensitive stability indicating spectrofluorimetric method. Results: Two highly sensitive spectrofluorimetric approaches were conducted for the assay of adapalene (ADP) in its gel. In the first approach, ADP exhibits an intense native fluorescence at 389 nm after excitation at 312 nm using borate buffer (pH 7.0)/ethanol system. This approach was successfully applied for routine analysis of ADP in its gel and ideally suited to the in vitro diffusion test. To elucidate the inherent stability of ADP, bulk sample was subjected to different stress conditions as specified by ICH guidelines. The acidic and oxidative degradation products were resolved from the intact drug using second and first derivative synchronous fluorimetry at 346 and 312.45 nm, respectively (the second approach). The synchronous fluorescence was scanned at Δ λ of 80 nm in case of acidic degradation and at Δ λ of 100 nm in case of oxidative degradation. Good linearity was obtained for ADP over the range 2.0-14.0 ng/mL with good correlation coefficient 0.999 in each approach. The approaches were carefully examined in terms of linearity, accuracy and precision. They were suitable for routine quality control laboratory. Moreover, the stability-indicating power of the second approach was ascertained via forced degradation studies. Conclusions: The proposed approaches were validated and successfully applied for the quantitative assay of a small concentration of ADP in its pharmaceutical gel. The conventional spectrofluorimetry was ideally suited for in vitro diffusion test. Stability studies were also conducted using different forced degradation condition according to ICH recommendation.